Understanding Lead-free soldering

Lead-free soldering

Understanding Lead Free

1.Solving the Plumbum (Pb) conundrum

Used in ancient times, lead (or ‘Plumbum’, Pb, in Latin) is a metal with a long application history. In fact, the word ‘plumber’ is derived from the Roman use of lead pipes for carrying water. However, when absorbed by the human body, lead is a cumulative poison detrimentally affecting health in several ways. And in children, lead can impede brain development, making them particularly vulnerable to lead poisoning.

Today, lead piping is obsolete and the largest use of lead is in car batteries. But some semiconductor packages and solders used in electronic circuit boards do contain lead. Though these are small quantities, the sheer volume of electronic consumer goods produced today means there is a risk of lead contaminating drinking water1) 2), plants and animals – for example, by acid rain filtering through landfill sites – and that is a concern to all of us.

WEEE directive: Proposal for a Directive of the European Parliament and the Council on waste electrical and electronic equipment, 2000/C 365 E/12.

Recycling and careful disposal go some way to addressing the lead health hazard but at NXP, we believe prevention is better than cure, i.e. making the manufacturing process Pb-free. We are actively engaged in researching new soldering materials, processes and package-terminal platings with the aim of making our broad product portfolio completely Pb-free in the near future. And in some products, such as the majority of our DIP, SIL and QFN packages, we’ve been Pb-free for many years.

As a global company committed to environmental care, we cooperate with leading suppliers and customers on a wide range of ‘green’ issues. It’s a commitment we take seriously, so it’s not surprising that the first factory in the world to receive ISO 14001 certification was one of ours.

Moreover, our Eco Vision program and Eco-Design philosophy ensure that better products and more environmentally-friendly manufacturing processes lie ahead. And that includes both Pb- and halogen-free products.

‘Profitable green’ is the name given to NXP Semiconductors’ approach to environmental issues. We aim to reduce the environmental impact of our products while providing customers with leading-edge technology at the lowest possible cost. One example of this is our ‘green’ plastic packaging. Used in all our new LQFP and TQFP packages, this plastic significantly increases product lifetimes at high temperatures, yet it does not contain brominated flame-retardants. Likewise, on the road to a Pb-free future, NXP Semiconductors is leading the way.

2.The need to be Pb-free

Throughout the electronics industry, the desire for Pb-free electronics is a hot topic. Customers and manufacturers alike are keen to ensure that new regulations in Europe, the US and Far East are complied with, sooner rather than later:

In Europe, an EU directive on Restrictions on the use of Hazardous Substances (RoHS) includes a requirement to eliminate Pb in electronics, in all but special applications, by July 1st, 2006

In China, the government is working on bringing in similar legislation to the Europeans

In the United States, laws banning or restricting the use of Pb are coming for many products and there is an increasing demand for a total ban.

The electronics industry is now focused on Pb-free assembly processes and the issues concerning the higher temperatures needed for both reflow and wave Pb-free soldering. By investing heavily in our own research organization, we have developed cost-effective Pb-free manufacturing processes that guarantee the reliability of our components. Moreover, we’re being proactive in also looking into solutions for devices where Pb is contained inside the package: replacing Pb-based solders in some Multi-Chip M odules/SiPs with new glues, for example.

3.Legislation

Formal guidelines have been introduced gradually over several years by various bodies, substantiating NXP’s own drive to eliminate lead from electronic products. Two of the most recent and important directives – WEEE (Waste from Electrical and Electronic Equipment) and RoHS (Reduction on Hazardous Substances) – impose better control over waste management of electronic equipment. Of most relevance to component manufacture, the RoHS establishes phase out dates for Pb – along with several other materials – from electronic products. This means by July 1st 2006, amounts of lead and the other named materials contained in electronic products must be below certain limits (unless exempt by special rule).

4.Tackling the issues

Understanding all the potential risks of going Pb-free is the key to ensuring our future success. By thoroughly addressing the following issues, we’re confident that a Pb-free future is just around the corner:

Our strategy to expand our range of Pb-free packages comprises a thorough investigation to tackle all of the above issues, ensuring full compatibility with new soldering processes, and to implement Pb-free materials for the terminals and solder balls on all packages.

5.Industry cooperation

Present soldering and plating technologies using Pb are well established throughout the semiconductor industry. Likewise, the procedures and standards for evaluating quality and reliability are recognized worldwide. With a large variety of new Pb-free technologies under investigation, NXP Semiconductors has been working together with other leading manufacturers. This is to ensure the methodologies for measuring solderability, heat resistance and whiskering are consistent, to fairly assess all competing technologies. For further details about this cooperation, please see our website.

Though our Pb-free technologies are based on our current research and experience, we are committed to high performance, quality and reliability, and we’ll ensure that our products will continue to delight customers.

6.Pb-free terminals

With many years of experience in using pure tin to coat semiconductor package leads/terminals, we know that tin is the logical choice for a drop-in replacement for those devices currently using SnPb-alloy coatings. We have studied whiskering with tin finishes, focusing on the leads of surface-mount devices and the results proved successful. Extensive testing for all packages is now completed.

Pure Sn, NiPdAu and SAC offer a very compatible replacement for SnPb alloys and consequently they are our preferred solution. In exceptional circumstances when pure tin is not feasible, we will offer alternative alloys. For the contacts of Ball-Grid Arrays (BGAs), we selected SnAgCu alloy (SAC).

7.Pb-free soldering process

Based on current research, we advise using SnAg3.8Cu0.7 (SAC) eutectic solder for general-purpose applications. In wave soldering, this would entail a bath temperature of 260 °ree;C and a contact time of about 3 seconds. Similar alloy would also be used for reflow soldering. Our temperature profile (see below) for moisture sensitivity characterization is based on the IPC/JEDEC joint industry standard: J-STD-020C.

Package Body Thickness

Package Volume <350 mm3

Package Volume 350 to 2000 mm3

Package Volume >2000 mm3

< 1.6 mm

260°C

260°C

260°C

1.6 to 2.5 mm

260°C

250°C

245°C

> 2.5 mm

250°C

245°C

245°C

Our current research indicates that higher soldering temperatures affect a number of surface mount devices. It is advised to keep the package top-body temperature at 245 oC max. Work continues to improve the resistivity of the products to high soldering temperatures. In addition, NXP Semiconductors is assessing mechanical stress and fracturing in ceramic and glass components – SAC is stronger than Pb-based solders.

8.Roadmap

NXP Semiconductors is now fully compliant with new regulations – well ahead of the European legislation. To check if an individual part is Pb free, enter a part number in the search box. For our roadmap overview see our lead-free package matrixpage. Please note that certain applications are exempt by RoHS regulations and so lead-containing products may be used in these areas.

9.Logistics, Identification and Labeling

Special product labeling will be used to identify Pb-free shipments. There are 3 ways for identification

A special Pb-free symbol on reel, bag and PQ box (and a different symbol for RoHS exempt packages) Lead-free products are not mixed with lead containing products within a single packing quantity (PQ).

Part numbers will remain the same (except new BGA parts, because of their non-backward compatibility) since most customers want part numbers to be kept consistent. There are several reasons for this, such as planning and capacity considerations, the need to control old parts (and avoid stock that needs scrapping), and the desire to reduce the logistics consequences for the supply chain.

All new Pb-free products are backward production compatible (except BGAs) meaning that they can be used in both a Pb-free and a Pb-based soldering process, which simplifies the manufacturing choices. As with any change it may be necessary to re-optimize the soldering processes.